Method of and apparatus for logging while drilling

ABSTRACT

A method and apparatus is described for monitoring at a remote location downhole conditions encountered while drilling a well. A sensed downhole condition is represented by a binary coded acoustic signal which is transmitted by way of a liquid path, provided by drilling liquid, to the surface of the earth. The acoustic signal, whose phase state represents bit values, is detected at the surface and decoded by way of a coherent system. Coherency is provided by deriving from the received signal a reference signal which is compared with the received signal to produce an output representative of the sensed condition.

United States Patent 1191 Patton Jan, 29, 1974 METHOD OF AND APPARATUS FOR LOGGING WHILE DRILLING Primary Examiner-Benjamin A. Borchelt Assistant Examiner-N. Moskowitz [75] Inventor Bobbie Patton Dallas Attorney, Agent, or Firm-Andrew L. Gaboriault et a]. [73] Assignee: Mobil Oil Corporation, New York,

NY 57 ABSTRACT [22] Flled: 1971 A method and apparatus is described for monitoring [21] A N 213,061 at a remote location downhole conditions encountered while drilling a well. A sensed do'wnhole condition is represented by a binary coded acoustic signal which is [52] US. Cl. 340/18 LD, 340/18 NC S transmitted by way f a li id th, provided by dril- [51] [Ill- Cl G011 1/40 ling liquid, to the Surface of the earth The acoustic [58] Field of Search 340/18 P, 18 LD Signal, whose phase State represents bit values, is

tected at the surface and decoded by way of a coher- [56] References C'ted ent system. Coherency is provided by deriving from UNITED STATES PATENTS the received signal a reference signal which is com- 3,622,971 11/1971 Arps 340/18 P pared with the received signal to produce an output 3,015,801 1/1962 Kalbfell 340/18 P representative of the sensed condition. 3,309,656 3/1967 Godbey 340/18 NC 2,700,131 1/1955 Otis et al. 340/18 P 13 Claims, 8 Drawing Flgures MULTIPLEX 2 ENCODER PAR/TY F GEN.

r @i b 88 (1- I j" /C 9 MASTER SYNC CLOCK COUNTER g WORD 77 5 7 GEN. I00 74 L 86 57 E 85 d 54 l TRANS. 102 104 d v4= 96 94 I 1 56 II vISAAgIPLE I H) 1 HOLD INDUCTION INVERTER C 98 1/3 P /93 T 1 MOTOR I I I V2 I B s l I MULT 103 I I 92 I l i ii m J PULSE GEN.

PAIENIEU I974 3.789.355

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PATENIEBJANZBISH SHEET 2 0F 7 GEAR REDUCTION UNIT ELECTRONICS SECTION GENERATOR mamm 3.789.355

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PATENIED 3,789,355

SHEET W 7 eJlll|llIlllll J L L I J L L I L I I L I L I dLlllllllllll Ll l l L I I J J J J Q D bfl num C JTRHWL "A;

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PuLSE GEN. PuLSE nn Lu I56 GEN. g I58 LU I T sAIvIPL E If BIT 62 Q j 68 a POLAR/TY g9 BIT VALUE mm 5 O D DETECTOR GEN. C I Q: \I E I- :1 3T I- SAMPLE 2 hh U GATE kk a M FILTER D 1 HOLD I70 I I63 DISCRIMINATOR NEGATIVE TRIG. BIT PHASE PULSE GEN. dd PHASE 2 ERROR DETECTOR METHOD OF AND APPARATUS FOR LOGGING WHILE DRILLING BACKGROUND OF THE INVENTION This invention relates to the logging of wells during drilling and more particularly to the telemetry of data relating to downhole conditions by means of an acoustic signal transmitted through drilling liquid within a well while concomitantly drilling the well.

It has long been the practice to log wells, that is, to sense various downhole conditions within a well, and concomitantly therewith transmit the acquired data to the surface. Well logging operations performed by service companies today utilize wireline or cable-type logging procedures. In order to conduct the operations, drilling is stopped and the drill string removed from the well. It is costly to stop drilling operations in order to log. The advantages of being capable of logging while drilling are obvious. However, the lack of an accept able telemetering system has been a major obstacle to a successful logging-while-drilling operation.

Various telemetering methods have been suggested for use in logging-while-drilling procedures. For exam ple, it has been proposed to transmit the acquired data to the surface electrically. Such methods have in the past proven impractical because of the need to provide the drill pipe with a special insulated conductor and means to form appropriate connections for the conductor at the drill pipe joints. Other techniques proposed for use in logging-while-drilling operations involve the transmission of acoustic signals through the drill pipe. Exemplary of such telemetering systems are those disclosed in US. Pat. Nos. 3,015,801 and 3,205,477 to Kalbfell. In the Kalbfell systems, an acoustic energy signal is imparted to the drill pipe and the signal is frequency modulated in accordance with a sensed downhole condition. Frequency shift keying is employed to transmit the acquired data in a digital mode. Yet other telemetering procedures proposed for use in loggingwhile-drilling systems employ the drilling liquid within the well as the transmission medium. Of these perhaps the most promising is the technique described in US. Pat. No. 3,309,656 to Godbey. In the Godbey proce dure, an acoustic wave signal is generated in the drilling liquid as it is circulated through the well. This signal is modulated in order to transmit the desired information to the surface of the well. At the surface the acoustic wave signal is detected and demodulated in order to provide the desired readout information.

SUMMARY OF THE INVENTION In accordance with the present invention there is provided a new and improved logging-while-drilling process wherein telemetry of information to the surface of the well is accomplished by phase modulation of an acoustic signal. In carrying out the invention, an acoustic signal is generated within the drilling liquid within the well and transmitted upwardly through the drilling liquid to a remote uphole station. One or more downhole conditions are sensed and the acoustic signal is modulated in response to such sensed conditions by varying the phase of the signal between a plurality of phase conditions to produce an m-ary encoded signal. The encoded signal is received at the uphole station and then demodulated or decoded to produce readout functions corresponding respectively with the phase conditions and thus the sensed conditions.

In a further aspect of the invention there is provided a new and improved downhole logging tool for use in the telemetry of information by phase shift keying of an acoustic signal in a liquid medium within a well. The logging tool comprises an elongated housing adapted for insertion into a well. The housing has a passage therein through which liquid within the well may flow. An acoustic generator supported by the housing functions to periodically obstruct the passage in order to impart an acoustic energy signal to the adjacent fluid. A plurality of logging transducers supported by the housing produce analog signals representative of sensed downhole conditions. The transducer signals are sequentially applied by means of a multiplexer to an analog-to-digital converter where the analog signals are converted to serial digital signals. A control unit operates the generator at a selected carrier frequency and responds to the digital signals to change the phase of the acoustic signal. In one embodiment this is accomplished by momentarily changing the frequency of the generator in order to shift the phase of the acoustic signal between at least two phase conditions.

In yet another aspect of the present invention there is provided a receiving system for recovering the acoustic signal from the drilling liquid and demodulating the signal to obtain the information carried thereby. The receiving system is coherent-that is, it derives the demodulating signal from the received signal. The receiving system comprises an acoustic transducer which is responsive to the acoustic signal within the drilling liquid to produce an output signal [representative of the phase and frequency of the signal. The system further comprises a generator which produces a control signal whose phase is controlled so to be substantially constant and to match one phase state of the output signal from the transducer. The transducer output signal and the control signal are compared in a phase detector which produces a plurality of readout functions corresponding respectively with a plurality of phase relationships between the control signal. and the transducer output signal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. ll illustrates a well drilling system equipped simultaneously to drill and to log;

FIG. 2 is a cross section of the logging sonde;

FIG. 3 is a block schematic of the downhole encoding and transmitting equipment;

FIGS. 4 and 5 are waveforms associated with the operation of the equipment shown in FIG. 3;

FIG. 6 is a block schematic of the uphole receiving system; and

FIGS. 7 and 8 are waveforms associated with the operation of the system of FIG. 6.

DESCRIPTION OF SPECIFIC EMBODIMENTS In accordance with the preferred embodiments of the invention as described in more detail hereinafter, an acoustic signal is transmitted through the drilling liquid employed in normal drilling operations. As the well is drilled, at least one downhole condition within the well is sensed and a signal, in most cases analog, is generated which is representative of the sensed condition. An analog signal is converted to a serial digital signal. The acoustic signal generated within the drilling liquid is modulated in response to the digital signal by correlating the phase of the acoustic signal during sequential time periods corresponding to the digit intervals of the digital signal with a plurality of phase conditions representative of the respective digit values of the digital signal. The acoustic signal is received at an uphole station and demodulated in order to produce appropriate readout functions corresponding with the respective phase conditions. These readout functions then may be applied to appropriate utilization means such as recording and/or data processing systems, such as computers, from which desired information may be derived.

Turning now to FIG. 1, there is illustrated a well which is being extended through the earths crust by means of a rotary drilling technique. Drilling operations are carried out utilizing a drill bit 12 attached to the lower end of a drill string 14. The drill string terminates at its upper end in a kelly 16 which is polygonal in cross section and extends through a rotary table 18 on the floor of the drilling rig. The kelly is received in the rotary table in a slidable torque-applying relationship by means of a kelly bushing (not shown). The rotary table is powered by a prime mover 20 through a suitable drive mechanism such as a chain drive 21 as will be readily understood by those skilled in the art. The drill string is suspended from the derrick structure of the drilling rig bymeans of a hook 23 which is attached to a traveling block 24. The traveling block in turn is suspended by a suitable cable arrangement 26 from the crown block (not shown) which is located in the upper portion of the derrick structure. The kelly is connected to the hook through a rotary swivel 28 which'permits rotation of the drill string relative to the hook and traveling block. Drilling liquid from a container 30 (commonly called a mud pit) is circulated by a pump 31 through a conduit 32 into the swivel 28 and thence downwardly through the interior passage of the drill string to the bit 12. The drilling liquid then passes outwardly into the wellbore through appropriate ports in the drill bit and is circulated to the surface of the well through the annulus between the drill string and the wall of the well. At the surface the mud is withdrawn from the annulus through a conduit 33 and recirculated to the mud pit 30. The conduit 32 is equipped with a desurger system 34 which functions to suppress undesirable noise in the drilling liquid introduced by pulsations produced by the pump 31.

Typically, the drilling liquid takes the form of a mud, that, is, a liquid having solids suspended therein. These solids function to impart desired rheological properties to the drilling liquid and also to increase the density thereof to a value adequate to provide sufficient hydrostatic pressure at the bottom of the well. In some instances, a relatively clear liquid may be employed whieh'contains little or no suspended particulate materials. The drilling liquid'may be either an aqueous-base liquid or an oil-base liquid.

Located within the drill string 14 near the drill bit is a downhole logging tool 36 which comprises one or more logging transducers for sensing downhole conditions and an acoustic generator for imparting an acoustic signal to the drilling liquid. Typically, the logging tool will be provided with transducers to measure a number of downhole conditions such as weight on bit, torque at the bit, pressure drop across the bit, pressure in the annulus, formation pressure, vibration, drilling mud temperature, and natural gamma ray count. The

acoustic generator employed may be of any suitable type which will impart a pressure wave signal to the drilling liquid which is of sufficient amplitude for transmission to the desired uphole location. A particularly appropriate generator for use in carrying out the present invention is a rotary valve transmitter of the type disclosed in the aforementioned patent to Godbey.

In carrying out the present invention, an acoustic signal is imparted to the drilling liquid by means of the acoustic generator within tool 36 and thence transmitted upwardly through the drilling liquid. This signal is modulated in response to a downhole condition sensed by a logging transducer by varying the phase of the acoustic signal. At the surface of the acoustic signal is recovered from the drilling liquid by means of one or more receiving transducers and converted to an electrical output signal. For example, a transducer 38 may be mounted on the upper section of swivel 28 as illustrated in FIG. 1. The signal from transducer 38 is applied to an uphole receiving system 40 where it is demodulated to produce the readout functions representative of the measured downhole conditions.

In accordance with the present invention it is preferred to employ an m-ary pulse code format, where m equals 2, 4, 8, in which the acoustic signal generated downhole is phase shift keyed to transmit the desired information in a serial digital format. For example, th phase conditions employed to impart intelligence-to the wave may be the phase state of the signal, i.e., the phase angle of the signal relative to a reference wave. Thus, for a binary code (m=2) an inphase state (phase angle of zero) may be employed for one bit value and a phase reversal state (phase angle of 180 degrees) may be employed for the other bit value. For a base four code (m=4) the phase conditions corresponding to the respective digit values may be phase states of 0, 90, 180, and 270 degrees. Alternatively, a nonreturn to zero scheme may be employed in which the phase conditions corresponding to the several digit values are several phase states of the signal relative to the phase state of the signal during a previous digit interval. For example, for a binary coded system, a phase continuity from one digit interval to the next may be indicative of a zero bit value whereas a reversal of phase from one digit interval to the next may be indicative of a bit value of one. For a ternary coded system the digit values may be indicated by a constant phase relationship from one digit interval to the next for one digit value, at degree phase shift for another digit value, and a 240 degree phase shift for the third digit value. vThe nonretum to zero coding technique is preferred for reliability in decoding inasmuch as bit valueis indicated by phase change. The invention will be described in detail with reference to a binary nonreturn to zero code.

The logging tool 36 is shown in more detail in FIG. 2. Tool 36 comprises elongated inner and outer housings 42 and 44 which define an annulus 45 through which the drilling liquid passes. The upper and lower ends of the outer housing are threaded for connection in the drill string. As illustrated, the logging tool is composed of three basic sections 46, 48, and 50. Associated with the upper section 46 is an acoustic generator or transmitter 54 comprising a slotted stator 52 and a rotor 54a with complementing slots which is driven relative to the stator. The rotor is shown in the open position in which slot 54b in the rotor and slot 52a in the stator, respectively, are in communication. A prime mover 56 applies power for rotation of the rotor 54a through a gear reduction unit 57. As is evident from an examination of FIG. 2, drilling liquid will pass through the slots in the rotor and stator. As the rotor is driven by the prime mover, the liquid stream is repeatedly interrupted to impart the desired acoustic wave signal to the drilling liquid. The diameter of rotor 54a is slightly less than the inner diameter of housing 44. Thus, some drilling liquid will pass around the rotor and through the slots in the stator when the rotor is in a closed position.

A tachometer 58 is mounted on the lower end of the motor shaft in order to provide a signal which is representative of the speed of the motor, and thus the speed at which valve 54 is driven. As described hereinafter, the tachometer signal is employed in a feedback loop to control the frequency and phase of the acoustic signal. Tachometer 58 comprises a coil 59 and and rotor 60 which is lugged to the motor shaft. Rotation of element 60 relative to coil 59 induces an alternating voltage signal in the coil of a frequency proportional to the operating speed of motor 56. The signal from coil 59 is then applied to conventional pulse shaping and frequency dividing circuits (not shown) to provide a pulse signal utilized to control the speed of the motor.

The intermediate section 48 of the tool is sealed off from the upper and lower sections by means of bulkheads 62 and 63 through which electrical leads pass. This section contains the electronic components associated with the logging tool, including those for receiving the output signals from the several logging transducers utilized to sense downhole conditions. A collar 64 surrounds housing 44 to provide an outer compartment 65 within which logging transducers may be located.

The lower section 50 of the logging tool houses a generator 68 which provides electrical power for the motor 56. Mounted within this section is an electrical generator 68 which is driven by a turbine 70. The turbine 7tll is rotated by the drilling liquid which flows downwardly through the tool through the annulus 45 between the inner and outer housings 42 and 44. For a more detailed description of the mechanical characteristics of the logging tool shown in FIG. 2, reference is made to the aforementioned patent to Godbey.

Turning now to FIG. 3', the major components of logging tool 36 are illustrated in block schematic form. This system includes a sync-word generator 74l and a plurality of logging transducers D D 1),, for sensing downhole conditions such as those described above and producing outputs representative of such conditions. The outputs (typically analog voltage signals) from units D D D are applied through a multi' plexer 80 to a coder 82. The multiplexer functions to apply the analog signal to coder 82 in any suitable sequence pattern. The reference character SW identifies a channel of the multiplexer 80 associated with a sync word introduced in another part of the system from generator 74. If desired, the multiplexer may be equipped with sufficient channels so as to provide for more frequent sampling of one or more signals from units D D D For example, the signal from transducer or unit D may be applied to two input channels in the multiplexer so that this parameter is transmitted twice for each multiplexer cycle.

The coder 82 is an analog-to-digital converter which produces a digital word in response to each analog signal from transducers or detectors D D D The output from coder 82 is applied to an encoder 841 by way of an lO-element gate and the output from sync-word generator 74L is applied to encoder 84 by way of lO-element gate 76. Encoder M is a parallel-in series-out shift register which functions to convert the parallel signal from coder 82 and from sync-word generator 74 to a serial digital signal which is then applied sequentially to a motor control unit 85. The motor control unit, multiplexer, coder, and encoder are controlled for synchronous operation by a master clock 86.

Sync-word generator 74 is a hardware unit that outputs on command from master clock 86 and counter 87 a predetermined word or words utilized to identify the beginning of a frame of data words.

Preferably, the system is provided also with a parity generator 8% which functions to add a parity bit to the word output of encoder Ml, thus providing a parity check for each word transmitted. In operation, the parity generator produces a parity hit of one state in response to an odd number of bits in a word of a given state and a parity bit of another state in response to an even number of bits in the word of the given state. For example, if an odd parity check is employed for bit values of one, the parity generator 88 will count the num ber of ls in the word and make the eleventh bit a I if the number of ls in the first ten bits is even and a 0" if the number of ls in the first ten bits is odd. Thus, each word frame transmitted will contain an odd number of l s, thus providing a check for transmission or reception errors.

The motor control unit d5 functions to maintain a phase lock transmission mode by comparing a feedback signal from the tachometer with the primary timing signal from the clock 86. In addition, motor control unit functions to execute a phase reversal of the acoustic signal in response to each signal l command from the encoder 84. In both instances, control voltages are applied from unit 85 to voltage-controlled unit 9% which supplies power to a two-phase induction motor 56. The motor 56 is, as described in FIG. 2, mechanically connected to drive the transmitter 54-.

The operation of the motor control unit 85', and associated circuits, in carrying out the aforementioned functions will be described with reference to the wave forms shown in FIGS. 4i and 5. The several waveforms illustrated and the respective points at which they appear in the circuitry of FIG. 3 are designated by reference characters in FIGS. 4 and 5. For the purpose of this description, it is assumed that the transmitter 54 is a lO-slot rotary valve of the type described previously which is operated at an acoustic carrier frequency of 25 hertz, equivalent to a transmitter speed of rpm. In one embodiment where the gear reduction unit 57 had a ratio of 25:1, the motor 56 was operated at a speed of 3750 rpm to produce from transmitter 5d an output at 25 hertz. In addition, the encoder output was framed to produce 1 l-bit (10 information bits plus 1 parity bit) binary words.

In operation, the master cloclc as and counter d7 provide timing signals a, b, c, d, and g (FIG. 5) to the' multiplexer, coder, encoder, motor control unit, and sync-word generator, respectively. In addition, the pulse generator 5% produces a train of signal pulses e which is applied to the motor control unit. The repetition rate of pulses e is a measure of the speed of motor 56. The generator 5dr: includes the tachometer 5% (FIG. 2) and related pulse shaping circuits. The frequency (pulse repetition rate) of the primary timing signal d is 50 pulses per second. Signal e likewise will have a repetition rate of 50 pulses per second when the transmitter 54 is operating at 25 hertz.

The various signals or pulses a, b, c, d, e, and g occur at repetition rates related in the following manner:

where, in the specific embodiment being described,

f, sonic frequency 25 hertz B number of bits per word l l P periods or cycles per bit 25 W number of words per frame 16 The speed of motor 56 is determined by the amplitude of signal output from unit 85. The output in turn is a function of the summation of four (4) voltages; V,, the value of which is determined by the phase relationship between pulses d and e; V a constant amplitude d-c voltage set at a value determined to produce a 25 hertz acoustic signal; V;,, a constant amplitude d-c voltage (a negative bias whose value is equal to positive V when pulses d and e are l80 degrees out of phase); and V.,, a phase reversal voltage, generated to change the speed of motor 56 in order to reverse the phase of signal generated by transmitter 54.

The voltages V and V, are shown provided, respectively, from batteries 91 and 93. The voltage V is derived in the following manner. Signal d from the clock is applied to a bistable multivibrator 92 and to a linear integrator 94. The signal e is also'applied to multivibrator 92 and to hold circuit 96. The clock signal d operates to turn the multivibrator 92 to the ON state where it applies a constant amplitude d-c voltage V to integrator 94. The signal e from the pulse generator 58a operates to return the multivibrator to the OFF state. Thus, the output from multivibrator 92 is a pulse train of constant amplitude, variable duration pulses in which the pulse duration is proportional to the interval between the clock pulse d and pulse e. This pulse train is applied to the integrator 94 which produces a voltage signal whose amplitude is proportional to the time duration of the respective pulses applied to it. The integrator holds the integrated value until it is reset to zero by the next-occurring clock pulse d, whereupon it is conditioned to respond to the succeeding pulse from multivibrator 92. Each final, integrated value at the integrator is sampled and held by sample and hold circuit 96 which functions in response to pulse e. Accordingly, a voltage representing the phase difference between pulses d and e is held from one pulse e to the next. This voltage, the voltage V and voltages V and V are applied to a summing amplifier 98.

The output from amplifier 98 is applied to motor power supply 90 to change the frequency of its output and thus control the speed of induction motor 56. More particularly, the power supply 90 comprises an inverter 102, a voltage-controlled oscillator 104, and a source of d-c illustrated as battery 103. The inverter 102 translates the d-c to a-c at a frequency of signal from oscillator 104, which frequency is determined by the output from amplifier 98.

As thus far described, the motor control circuitry drives the motor 56 and therefore the acoustic transmitter 54 in a phase locked mode in which the clock pulses d are 180 degrees out of phase with respect to pulses e from the pulse generator 58a. in this mode the positive voltage V from sample and hold circuit 96 is equal in amplitude to the negative voltage V and the motor speed is determined by voltage V A change in motor speed will change the phase relationship between pulses d and e and in turn vary the amplitude of voltage V,. The change in V an error signal, will be effective to rebalance the system and bring the motor back to its proper speed and selected phase.

The operation of the transmitting system in effecting phase shifting of the acoustic signal will now be described. The rate or frequency of signal a applied to the multiplexer from counter 87 is one pulse per 11 seconds. Upon the application of the signal pulse a, the multiplexer steps from one channel to the next and applies a selected analog value to the coder 82. The frequency of the signal b to the coder is also one pulse per 1 1 seconds. This signal is delayed slightly with respect to signal a in order to accommodate the stepping time of the multiplexer and the conversion time of the coder. The coder converts the analog value to a [0-bit binary word representative of the applied analog value. The frequency of the signal 0 applied to the encoder 84 is one pulse per second and this signal is also shifted in phase with respect to signal b to provide a suitable time delay. The encoder responds to each applied pulse to generate a command signal j, either a l or a 0. The motor control unit 85 responds to the occurrence of each 1 command to reverse the phase of the acoustic signal and to the occurrence of each 0" command to continue the acoustic signal in the same phase.

The bit interval A is a 1 bit. As a result, the encoder 84 responds to generate pulse j which is applied to waveform generator which produces a square wave voltage k The summing amplifier 98 output now increases and is effective by way of supply 90 to increase the speed of motor 56 to advance the phase of the acoustic signal. The motor speed and the attendant acoustic signal are illustrated waveforms I and m, respectively. As shown, the motor speed increases substantially linearly as the voltage output from amplifier 98 increases. When the voltage k is suddenly decreased, the motor decelerates until it returns to the normal speed equivalent to the carrier frequency of 25 hertz. At this point, the signal is approximately degrees out of phase with respect to the previous phase state and the motor control unit will operate in response to the feedback signal from pulse generator 58a to achieve an exact phase locked condition.

The amplitude of square waves k is made large compared with the maximum output from integrator 94 and this nullifies the effect of the feedback control provided by pulses e, enabling the motor 56 to speed up and change phase. The generator 100, for purpose of this description, may be considered a part of the motor control circuit 85 though illustrated outside the dashed lines. The output of generator 100, though shown and described as a square wave, may be of any form suitable to change the speed of motor 56.

When the bit value as shown during interval B is a 0," the command signal from the encoder is simply the absence of a pulse and the acoustic generator continues to operate to produce an acoustic signal having the same phase state as in the previous bit interval A. The bit value during interval C is a l so the encoder 84 again responds to the pulse c to generate a pulse j which is received at generator 100. An output k is generated and the motor 56 is again accelerated and decelerated as indicated in order to reverse the phase of the acoustic signal.

The format in which the data is framed for transmission from the downhole sonde to the surface for detection is illustrated in FIG. 5. Beginning each frame is a sync word, followed by data words 1 through N. In the embodiment being described, there are data words: N 15. Examples are illustrated in the various data words of the value of the parity bit as being either 0, as in data word I, or a one, as in data word 2, in order that an odd number of one bits comprise each data word. Illustrated also is the occurrence of pulses a and b at the onset of each word. 4

Pulse g, however, occurs only once for each frame and is utilized to control the time occurrence of each sync word and also the opening and closing of necessary gates in order to apply the sync word to the encoder 84. The sync-word control pulse g is generated by the counter 87 and applied to the sync-word generator 74. Concurrently it is applied to a gate generator 77 which may be of the monostable, multivibrator type which outputs a gating waveform h as well as a gating waveform h, not illustrated but which is the complement of the waveform h. The waveform h is applied to the gate 76 to open it while the waveform h is applied to the gate 75 to close it. With gate 76 open and gate 75 closed, the output from the sync-word generator is applied directly to the encoder 84 and the output from the AD converter 82 isblocked from the encoder. After a time interval, predetermined to be one-word length in duration, the gate generator returns to its stable state and waveform h moves in a negative-going direction to close gate 76; and its complement waveform h, moving in a positive-going direction, is effective to open gate 75.

Another feature of the present system is the use of two sync words, one the complement of the other. These sync words are generated by the sync-word generator alternately. In other words, the first sync word would be used for the odd-numbered frames of data whereas its complement would appear at the beginning of the even-numbered frames. The sync word is illustrated in FIG. 5 in frame 1, and its complement is illustrated beginning at frame 2.

The length of each bit may be varied by adjusting the counter 87, for example, by turning the knob 87a, in order to increase or to decrease the number of cycles representing each bit. Where the signal-to-noise ratio is high, such as may be encountered during the shallow portion of the drilling program, the bit length may be one second or less. As the hole deepens, the signabtonoise ratio will decrease and it will be desirable to increase the number of cycles representing each bit or, in other words, increase the bit length. A change in the bit length will, of course, necessitate change in the rates of the various pulses, for example, the pulses a, b, c, etc., all in accordance with the relationships earlier set forth.

As noted previously, the transmitted acoustic wave signal is received at the surface by a transducer 38 which functions to convert the acoustic signal to a form suitable for demodulation. Transducer 33 may be of any appropriate type which responds to the acoustic signal to generate an output signal which is representative of the phase of the acoustic signal. By way of example, transducer 38 may be a piezoelectric crystal which generates an a-c signal of the same frequency as the acoustic signal.

The output signal from the transducer 38 is applied to a receiving system which generates from the output signal a control signal which remains in synchronism with one phase state of the received acoustic or output signal. The received signal is demodulated by comparing it with the control signal to detect phase shifts in the received signal and appropriate readout functions are generated which correspond with different phase relationships between the received signal and the control signal.

Referring now to FIG. 6, a preferred embodiment of the receiving system employed in the present invention is illustrated in block diagram. The waveforms asso ciated with the operation of the system are illustrated in FIGS. 7 and 8 with the waveforms and the points at which they appear in FIG. 6 designated by common reference characters. In order to simplify the description, the exemplary transmission mode .and parameters specified above with respect to the transmitting system will be assumed here. Thus, the carrier frequency of the received acoustic signal is 25 hertz and binary nonreturn to zero keying is employed. However, it will be recognized that the receiver may be employed with other suitable transmission modes such as those noted previously.

The received signal is applied from the output of transducer 38 to a low noise amplifier 1110 to avoid introducing noise distortion to the signal. The higher gain signal from amplifier 1110 is applied to a filter 1112, whose band pass is the reciprocal of the bit length utilized in order to attenuate noise outside the frequency of interest. Where the bit length is one (1) second, the band pass would be one cycle, for example, 24.5 to 25.5 hertz. From filter 112 the signal is fed through an automatic gain control amplifier EM in order to eliminate wide amplitude excursions in the signal and to provide a signal of relatively uniform amplitude p.

The signal p from amplifier 1114 is applied both to a synchronous phase detector M6 and to a phase-lock loop 117 in which a control signal z for detector I 116 is derived as described hereinafter. Phase detector llllb functions to compare the phase of signal p with the control signal z to produce a signal r representative of the phase relationship between the two signals. By way of example, phase detector 1111s may be a switching-type unit in which the control signal z functions to switch the signal p between positive and negative inputs of the detector. During the positive-going excursions of the control signal the output signal r of the phase detector is reversed in polarity with respect to signal p. During negative excursions of the control signal z, the output signal from detector 116 is the same polarity with respect to the signal p. Thus, when the transducer signal p and the control signal z are out of phase, as indicated in FIG. 7 by segments appearing in portion labeled Phase I, the output r from the detector 116 is positive. When the signal p and the control signal 1 are in phase with respect to one another as shown by segments appearing in portion labeled Phase II, the output from unit 116 is a negative half-wave signal.

In the phase-lock loop 117,.the control signal z for detector 116 is originated by a voltage-controlled oscillator 120. The oscillator 120 is initially adjusted by application of appropriate voltage values to produce pulses t at a repetition rate four times that of the frequency of the received acoustic signal p. In the embodiment disclosed the pulses t have a repetition rate of 100 hertz.

Feedback control for oscillator 120 is derived from the transducer output signal p. This signal is applied to a frequency multiplier in which the frequency is doubled to produce a SO-hertz signal s. By doubling the frequency, the phase state of signal s remains the same regardless of phase shifts which may occur in the transducer signal p.

Signal t from oscillator 120 is applied to a square wave generator 121, which may be a bistable multivibrator, to produce pulse train u having a frequency equal to that of signal s. Signals u and s are applied to a synchronous phase detector 126 which operates in the manner described above with reference to phase detector 116. The output from detector 126 is the wave train v whose average d-c component will be zero when the loop 117 is in phase lock. Under such condition the pulses u are 90 out of phase with signal s.

The signal v is utilized to control the frequency and phase of VCO 120. It is applied by way of amplifier 130 and filter 132 to VCO 120. The amplifier 130 has a preadjusted gain set in accordance with parameters of loop 117 to provide the necessary control from signal v. The filter 132 has a long time constant such that substantially only the d-c component of v, the error signal w, is fed VCO 120. This feedback arrangement assures that the output u from square wave generator 121 will be maintained in a predetermined phase relationship with signal s, that is, it will be 90 out of phase with signal s.

The operation of the feedback control is illustrated in FIG. 7 by an initial error condition and a change in the phase of signal p. Assuming that the signals s and u are other than in the 90 phase relationship, a negative component will be developed as shown by waveform w. The error is greatly exaggerated in the drawings for purpose of illustration. The negative signal w when applied to VCO 120 corrects the output such that signals s and u are brought into the desired phase relationship. When the downhole generator changes phase, an error signal is again developed, first moving in a negative direction and then in a positive direction. However, about the time the change in phase of signal p is completed, signals s and u are again in the desired phase relationship.

The wavetrain or signals u are applied to a pulse generator 135 which responds only to the leading edge of signals 14 to produce pulses x. The repetition rate of pulses'x is at twice the frequency of signals p, in this instance, 50 per second.

Remembering that the purpose of the circuitry is to produce signals z whose phase state will be constant and be one of the phase states of the signal p, it will be necessary to shift the phase of signals or pulses x. The actual phase shift introduced will be such that the shifted pulse will have a time occurrence corresponding with thezero crossing of signal p. The shift is introduced by delay circuit 136 whose output is a series of pulses y. i r

The pulses y are applied to square wave generator 138, a bistable multivibrator, to produce the square wave pulses z. The trainof pulses z are applied to the phase detector 116 and in the manner described above there are produced the summation signals r, the detected output, wherein a phase of one state of signal p will be represented by positive-goingsignals r and a phase of another state of signal p will-be represented by negative-going signals r.

It will be recalled that the amplifier is of fixed gain. Therefore in order to assure operation of the loop 117, the amplitude of signal p applied to the loop is held substantially constant. The amplitude control is provided by amplifier 114 and an associated automatic gain control circuit. The detected signals r are full wave rectified by rectifier and filtered by low pass filter 142 to produce a representation of the amplitude of signal p. The function, which may be monitored by meter 144, is applied to a differential amplifier 146 which compares the value of the function with the value of a reference voltage V from source 148. Changes in the value of the function about the value of V are fed to the amplifier 114 to control its gain and to hold the amplitude of signal p substantially constant. The amplitude or level at which signal p is held is determined by setting the amplitude or value of voltage V as by adjusting knob 149 on source 148.

At this stage useful data is developed in the formof signals r. This signal need but be decoded applying the principle that a change in phase (polarity) is a 1 bit and a lack of change in phase (polarity) is a 0 bit. The signal r may be recorded in visible form on a chart recorder (not shown) and decoded by an operator. However, automatic machine decoding is preferred especially into a format that will be machine readable. To this end, a bit lock or sync system is utilized.

As a first step in acquiring bit sync and detection, the signal z, at a frequency f, (the frequency of the signal p), is applied to a divider 150 which produces a pulse train aa at a repetition rate equal to the bit rate. In the present system the bit period is one (1) second and the duration of each of the pulses aa is one half a period or one-half second. The onset of pulses aa must, as will be apparent, coincide with the beginning of each bit period. Because the pulses z are independent of bit period and the onset, provision is made to adjust the onset of pulses an to coincide with the onset of each bit period. The adjustment is provided by adjustable delay 152 whose output is a train of pulses bbshown in FIG. 8 to be out of phase with pulses aa and having onsets coinciding with the beginning of bit periods shown as signal r.

With the occurrence of pulses bb properly adjusted the bit detection may be accomplished by starting the integration of thesignal r at a time corresponding with the onset of each pulse bb and terminating the integration at a time corresponding with the end of each bit period. The sampled value of each integration will be indicative of the value of the bit signal r and thus each bit will be properly identified.

In carrying out the integration-detection, the pulse train bb is utilized to trigger pulse generator 154 which responds to the reset or positive-going portion of each pulse bb to produce a train of short duration pulses cc. The pulses cc are applied to integrator 156, also referred to as the in-phase integrator, which is reset in response to the trailingedge of each pulse cc. A sample and hold circuit 158 responds to the leading edge of each pulse cc to sample the output of the integrator 156. The output of integrator 156 is the integrated value ee of the signal r applied to the input of the integrator. The output of the sample and hold circuit 158 is the waveform ff.

A l bit is represented by each zero crossing of the waveform ff. If, as utilized herein, a zero-crossing detector is utilized to generate the l and the bits, the waveform ff should be modified such that it has uniform maximum and minimum values, otherwise there may be false tracking by the zero-crossing detector. Accordingly, the waveform ff is applied to a bitpolarity detector 160, a saturation amplifier that is driven to saturation for each positive or negative value of the waveform ff. The result is the waveform gg which is applied to a bit-value generator 162, a zero-crossing detector whose output is the pulse train mm comprised ofl bits, represented by the presence of pulses, and 0 bits, represented by the absence of pulses.

The pulse train mm is applied to a utilization device 163 which may be a recorder or a digital computer either hardwired or of the general purpose programmable type. In either event it is desirable to provide information as to when to read the information in pulse train mm, that is, information as to time at which to look for a 0 bit or a l bit. This information is provided by pulses from the read generator 164. The read generator is a unit to which is applied the pulse train cc. The pulses are delayed a predetermined time to occur during the expected time occurrence of the bit pulses mm and the result is the pulse train nn.

All necessary information is provided by the pulse trains mm and rm to decode the data and provide measurement of downhole conditions. By utilizing a programmable computer, appropriate software may be provided to identify the sync words, announcing the start of each frame and to collate the data from each of the downhole data channels.

The amount of delay to be introduced to the pulses aa in order that the integrator 156 integrate signal r representing one bit at a time and avoiding overlap into preceding or succeeding bit information is determined by an arrangement utilizing an out-of-phase integrator 170. The integrator 170 has applied to its input the bit information as represented by waveform r and has an output when the system is properly adjusted as represented by the initial portion of wavetrain hh. As illustrated, the integrator 170 begins its operation at a time delayed approximately 90 from that of the in-phase integrator 156. The integrator 170 is set to integrate over a period of one bit length; therefore, its output at the end of an integrating period should be zero. Accordingly, the delay unit 152 is adjusted until the output from the integrator 170 is zero at the end of each period of integration. The adjustment is carried out by applying the pulses bb to the input of a pulse generator 172 which responds to the negative-going excursions or the trailing portion of the pulses bb to produce a series of short duration pulses dd. The integrator 170 is reset by the trailing portion or the negative-going portion of each pulse dd and begins the integration of the input waveform r to produce wavetrain hh. The wavetrain hh together with the bit information, the wavetrain gg, is supplied to a synchronous phase detector 174 operating in the same manner as the phase detectors 116 and 126 to produce an output represented by the wavetrain ii. The wavetrain ii is applied by way of normally open gate 176 to the input of a sample and hold circuit 178 which responds to the leading edge of the pulses dd to momentarily sample and then to hold the output or signal ii at a time corresponding with the end of the period of integration by integrator 170. With the system properly adjusted the sample and hold circuit 17% will read zero each time the wavetrain ii is sampled and its output can be represented as shown by the signal II. If the system is not tracking properly, an error signal will result at the output of the sample and hold circuit 178 which will be detected by meter 180. The meter will indicate the amount of error and the direction of error which can be corrected by adjusting the delay unit 152 as by moving knob 152a.

it will become readily apparent from the examination of the waveform r that there are conditions under which the output from the integrator 171) will be other than zero at the time it is to be sampled even though the system is properly adjusted. This condition exists when zero bits are being transmitted and received. Such condition is illustrated by the waveform hh attaining full negative value whenever a zero bit or a succession of zero bits are transmitted and received. An operator making the adjustments by observing the meter will learn that large excursions of the meter are to be ignored as merely representing the occurrence of a zero bit. As a matter of practice, the maximum error to be expected or to be read by the meter 1111 will be ap proximately one half the full integrated value of a received bit. Any indication by the meter 1W1 of a value less than one half the integrated value of a bit may be treated as an error and anything larger than the onehalf value is to be ignored.

The above logic in interpreting the output of a meter 1111 may be represented by hardware and the adjustment made automatically. This is accomplished by providing a discriminator 182 having applied to one input a reference voltage V having an amplitude equal to one half the integrated bit value. The wavetrain ii is applied to the other input of the discriminator. Whenever the amplitude of the signal or wavetrain ii exceeds the half value, the discriminator will apply a control pulse which will close gate 176. The end result will be the wavetrain kk where an operation of the discriminator to close the gate is represented by portions of the wavetrain kk appearing between the segments bounded by small arrows where the signal kk has been reduced to zero. With the discriminator operating as above described, the sample and hold circuit 173 will see a zero signal at each sampling interval unless the system is in fact out of adjustment. Let us assume that the train of pulses dd has been delayed an amount 8. In this event the operation of sample and hold circuit 17% will be delayed and it will see an output from the integrator. This output, identified by reference character 1911, will be applied by way of a low pass filter 192 to the delay unit 152 to change impedances therein to effect the necessary correction in the amount of delay being applied to the pulse train aa. If the error is in the other direction as illustrated by advance in one of the pulses d by an amount -8, the sample and hold circuit 178 again will see an output from the integrator 170. This output, represented by the negative-going waveform 1194i, will likewise be applied by way of the filter 192 to effect the necessary corrections in the delay unit 152. V

The above description of automatic feedback to acquire and maintain bit phase sync utilizes an analog feedback control signal which is amplitude proportional to the phase error 6 and is bipolar in accordance with the direction of the phase error. Other feedback mechanisms may be used. For instance, a purely digital scheme of either inserting extra pulses or deleting pulses at the input of divider 150 is effective in shifting the phase of the bit integrators. Delay 152 is used in this modification as an initial adjustment of the circuits to get exact phasing.

I claim:

1. In a logging-while-drilling system for transmitting downhole measured conditions to the surface of the earth during the drilling of a well utilizing a flowing drilling liquid including an acoustic generator having a movable member which when driven at a constant speed produces in the liquid a continuous acoustic wave signal having a first phase state and having a frequency proportional to the speed of movement of said movable member, driving means for said member, and a downhole transducer, the improvement comprising:

means responsive to the output of said transducer for momentarily changing the speed of said member and then re-establishing said constant speed to change the phase of said acoustic signal from the first phase state to a second phase state.

2. The system of claim 1 wherein said acoustic signal is coded in m-ary coded bit lengths, where m is any integer greater than l, means for initiating said bits at a selected rate, and means for varying the bit initiation rate to vary the time duration over which said acoustic signal representing each bit is generated.

3. The system of claim 2 where m 2, 4 or 8.

4. The system of claim 1 wherein the movable member for generating the acoustic signal comprises a rotating valve for interrupting liquid flow, means for generating pulses at a predetermined repetition rate to control the rotation of said valve, means responsive to the rotation of said valve for producing a signal representative of the rate of rotation of said valve, means for comparing said pulses and said signal to produce an error signal when said valve is operating at a phase other than a preselected phase, and means responsive to said error signal to adjust the rate of rotation of said valve to regain said preselected phase and return said error signal to zero.

5. The system of claim '1 including a plurality of means for sensing downhole conditions and for generating analog signals representative of said conditions, cycling means for multiplexing said signals, an analogto-digital converter for converting said analog signals into digital words represented by two states and l an encoder responsive to said digital words to produce an output signal only upon the occurrence of one of said states, means for introducing to said system a sync signal at the beginning of each cycle of said means for multiplexing, and wherein said means for momentarily changing the speed of said member responds to each output signal from said encoder for reversing the phase of said acoustic signal.

6. The system of claim 5 in which said means for introducing said sync signal introduces alternately two sync signals, one the complement of the other.

7. The system of claim 5 including means for generating said states at a selected rate, and means for varying the state generation rate to vary the time duration over which said acoustic signal representing each state is generated.

8. In the drilling of a well employing a drilling liquid and a drill string within the well, the method of logging said well comprising driving a movable member of an acoustic generator at a constant speed to produce in the liquid a continuous acoustic wave signal having a first phase state and having a frequency proportional to the speed of movement of said movable member, sensing at least one downhole condition and in response to said sensed condition changing the speed of said member and then re-establishing said constant speed to change the phase of said acoustic signal from the first phase state to a second phase state.

9. The method of claim 8 wherein said acoustic signal is modulated in a nonretum to zero mode in which phase conditions are different phase states of said acoustic signal with reference to the phase state of said signal during a preceding time interval.

10. In a well logging tool, the combination comprising:

an elongated housing adapted to be inserted into a well and into contact with liquid within said well,

said housing having a liquid flow passage therein,

an acoustic generator supported by said housing and including a movable member which when driven at a constant speed produces in the liquid a continuous acoustic wave signal having a frequency proportional to the speed of movement of said movable member,

means for regulating the operation of said generator at a selected frequency,

a plurality of transducers supported by said housing for producing analog signals representative of sensed downhole conditions,

means for converting said analog signals to serial digital signals,

multiplexer means for serially applying the signals from said transducers to said converting means, and

means for momentarily changing the speed of said member and then re-establishing said constant speed to change the phase of said acoustic signal in response to said digital signals.

11. The combination of claim 10 wherein said means for regulating the operation of said generator comprises:

clock means for generating a primary timing function representative of said selected frequency,

pulse generating means associated with said generator for generating a secondary timing function representative of the actual operating frequency of said generator,

means for comparing said primary and secondary timing functions, and

means for adjusting the operating frequency of said generator in response to the deviation of said secondary timing function from said primary timing function.

12. In a well logging system having a logging tool movable lengthwise through a well filled with liquid and including an acoustic generator for imparting to the well liquid an acoustic signal whose frequency is proportional to the speed of a motor driving the acoustic generator, and having one or more transducers for producing output signals representative of sensed downhole conditions, the improvement comprising:

a signal source of constant frequency,

signal means driven by said motor for generating within said well liquid an acoustic output signal having a frequency proportional to the speed of said motor,

means for producing a signal proportional to the speed of the motor,

means responsive to said constant frequency and to said signal representative of motor speed for producing a resultant output signal varying with phase differences between them,

means responsive to said last-named output signal for maintaining said motor at a constant speed with a resultant fixed-phase relationship between said constant frequency and said acoustic output signal,

an uphole transducer acoustically coupled to said well liquid for generating an output signal having the same frequency as that of said acoustic output signal,

an oscillator for producing a reference frequency,

means includinga phase detector for adjusting said oscillator for maintaining a fixed-phase relationship between that of said reference frequency and said acoustic output signal,

means responsive to each transducer carried by said logging tool for applying to said driving motor a sedata via a flowing drilling liquid between spaced points during the drilling ofa well utilizing an acoustic generator for producing a continuous wave having a movable member which when driven at a constant speed produces in the liquid a continuous wave signal having a first phase state and having a frequency proportional to the speed of movement of said movable member, driving means for said member, and a signal source, the improvement comprising:

means responsive to the output of said signal source for momentarily changing the speed of said driving means and then re-establishing said constant speed to change the phase of said acoustic signal from the first phase state to a second phase state. 

1. In a logging-while-drilling system for transmitting downhole measured conditions to the surface of the earth during the drilling of a well utilizing a flowing drilling liquid including an acoustic generator having a movable member which when driven at a constant speed produces in the liquid a continuous acoustic wave signal having a first phase state and having a frequency proportional to the speed of movement of said movable member, driving means for said member, and a downhole transducer, the improvement comprising: means responsive to the output of said transducer for momentarily changing the speed of said member and then reestablishing said constant speed to change the phase of said acoustic signal from the first phase state to a second phase state.
 2. The system of claim 1 wherein said acoustic signal is coded in m-ary coded bit lengths, where m is any integer greater than 1, means for initiating said bits at a selected rate, and means for varying the bit initiation rate to vary the time duration over which said acoustic signal representing each bit is generated.
 3. The system of claim 2 where m 2, 4 or
 8. 4. The system of claim 1 wherein the movable member for generating the acoustic signal comprises a rotating valve for interrupting liquid flow, means for generating pulses at a predetermined repetition rate to control the rotation of said valve, means responsive to the rotation of said valve for producing a signal representative of the rate of rotation of said valve, means for comparing said pulses and said signal to produce an error signal when said valve is operating at a phase other than a preselected phase, and means responsive to said error signal to adjust the rate of rotation of said valve to regain said preselected phase and return said error signal to zero.
 5. The system of claim 1 including a plurality of means for sensing downhole conditions and for generating analog signals representative of said conditions, cycling means for multiplexing said signals, an analog-to-digital converter for converting said analog signals into digital words represented by two states ''''0'''' and ''''1,'''' an encoder responsive to said digital words to produce an output signal only upon the occurrence of one of said states, means for introducing to said system a sync signal at the beginning of each cycle of said means for multiplexing, and wherein said means for momentarily changing the speed of said member responds to each output signal from said encoder for reversing the phase of said acoustic signal.
 6. The system of claim 5 in which said means for introducing said sync signal introduces alternately two sync signals, one the complement of the other.
 7. The system of claim 5 including means for generating said states at a selected rate, and means for varying the state generation rate to vary the time duration over which said acoustic signal representing each state is generated.
 8. In the drilling of a well employing a drilling liquid and a drill string within the well, the method of logging said well comprising driving a movable member of an acoustic generator at a constant speed to produce in the liquid a continuous acoustic wave signal having a first phase state and having a frequency proportional to the speed of movement of said movable member, sensing at least one downhole condition and in response to said sensed condition changing the speed of said member and then re-establishing said constant speed to change the phase of said acoustic signal from the first phase state to a second phase state.
 9. The method of claim 8 wherein said acoustic signal is modulated in a nonreturn to zero mode in which phase conditions are different phase states of said acoustic signal with reference to the phase state of said signal during a preceding time interval.
 10. In a well logging tool, the combination comprising: an elongated housing adapted to be inserted into a well and into contact with liquid within said well, said housing having a liquid flow passage therein, an acoustic generator supported by said housing and including a movable member which when driven at a constant speed produces in the liquid a continuous acoustic wave signal having a frequency proportional to the speed of movement of said movable member, means for regulating the operation of said generator at a selected frequency, a plurality of transducers supported by said housing for producing analog signals representative of sensed downhole conditions, means for converting said analog signals to serial digital signals, multiplexer means for serially applying the signals from said transducers to said converting means, and means for momentarily changing the speed of said member and then re-establishing said constant speed to change the phase of said acoustic signal in response to said digital signals.
 11. The combination of claim 10 wherein said means for regulating the operation of said generator comprises: clock means for generating a primary timing function representative of said selected frequency, pulse generating means associated with said generator for generating a secondary timing function representative of the actual operating frequency of said generator, means for comparing said primary and secondary timing functions, and means for adjusting the operating frequency of said generator in response to the deviation of said secondary timing function from said primary timing function.
 12. In a well logging system having a logging tool movable lengthwise through a well filled with liquid and including an acoustic generator for imparting to the well liquid an acoustic signal whose frequency is proportional to the speed of a motor driving the acoustic generator, and having one or more transducers for producing output signals representative of sensed downhole conditions, the improvement comprising: a signal source of constant frequency, signal means driven by said motor for generating within said well liquid an acoustic output signal having a frequency proportional to the speed of said motor, means for producing a signal proportional to the speed of the motor, means responsive to said constant frequency and to said signal representative of motor speed for producing a resultant output signal varying with phase differences between them, means responsive to said last-named output signal for maintaining said motor at a constant speed with a resultant fixed-phase relationship between said constant frequency and said acoustic output signal, an uphole transducer acoustically coupled to said well liquid for generating an output signal having the same frequency as that of said acoustic output signal, an oscillator for producing a reference frequency, means including a phase detector for adjusting said oscillator for maintaining a fixed-phase relationship between that of said reference frEquency and said acoustic output signal, means responsive to each transducer carried by said logging tool for applying to said driving motor a series of control pulses of magnitude and duration for producing transient changes in the speed of said motor to change by predetermined amounts the phase of said acoustic output signal relative to that of said reference frequency, and means including a phase detector responsive to said changes in phase due to said transient speed changes for producing output pulses corresponding with said control pulses.
 13. In a logging-while-drilling system for transmitting data via a flowing drilling liquid between spaced points during the drilling of a well utilizing an acoustic generator for producing a continuous wave having a movable member which when driven at a constant speed produces in the liquid a continuous wave signal having a first phase state and having a frequency proportional to the speed of movement of said movable member, driving means for said member, and a signal source, the improvement comprising: means responsive to the output of said signal source for momentarily changing the speed of said driving means and then re-establishing said constant speed to change the phase of said acoustic signal from the first phase state to a second phase state. 